As the radio spectrum becomes more crowded, spectrally efficient digital modulation schemes such as orthogonal frequency division multiplexing (OFDM) are replacing existing analog transmission schemes. This is equally true for the terrestrial amplitude modulation (AM) radio broadcast band, which until recently has resisted any move towards digital broadcast due to the huge costs involved.
For the AM radio broadcast band, there are several standards being considered for digital audio broadcast (DAB), all using OFDM. One of the main challenges to implementing digital audio broadcasting is that existing broadcast equipment was not designed for such a scheme. Unlike analog AM, which is a low bandwidth amplitude modulated signal, OFDM is a noise-like signal with significant amplitude and phase modulation.
Most AM transmitters use envelope elimination and restoration (EER) amplifier architecture, where the amplitude and phase components of the signal are amplified separately then recombined at the high power stage. The magnitude and phase component bandwidths are several times that of the input signal, and any filtering in the amplifier will result in spectral regrowth due to poor cancellation of this high frequency content.
In particular, in an EER amplifier, both the magnitude and phase-modulated carrier experience filtering. On the magnitude path, there will be a reconstruction filter to eliminate switching noise and harmonics from the power electronics. The phase modulated carrier has to pass through a network drive, which may be tuned to the carrier frequency. When the signals are recombined, the transitions in the two signals may not perfectly cancel, resulting in spectral regrowth.
Accordingly, there is a need to develop a method of signal correction processing that can be applied to a digital audio broadcast signal intended for transmission through an EER transmitter to reduce spectral regrowth.